Conservative groups are engaging in a fierce internal fight about the economic impact of the Senate's bipartisan immigration reform bill. The outcome of that debate could influence whether fiscally-minded GOP lawmakers decide to vote for the proposal.

On Monday, the Heritage Foundation released a report on the Senate's bill that claims it will cost taxpayers $6.3 trillion in government benefits provided to legalized immigrants over their lifetime as a group, a period that could span five decades.

You can read the full report here to see how Heritage arrived at its number.

Here's our short summary: The group, led by former senator and longtime foe of immigration-reform, Jim DeMint, views the bill through the lens of "the makers versus the takers."

In Heritage's view, there are million of immigrants who could benefit from legalization who have lower levels of education and work skills than the general population. Heritage acknowledges that if immigrants are legalized, they will earn higher wages and pay more in taxes. But that won't be enough to prevent them from becoming a drag on government services (aka "takers"), according to the study.

The study also suggests, they will end up taking more from the government in direct benefits, under programs like Medicaid, means-tested welfare, and even in police and fire services than they will add to the government in tax revenue.

Current immigration law, in which 11 million people are living in the shadows, is a "better deal for the taxpayer," according to the study's co-author, Robert Rector.

But conservative proponents of immigration reform see a litany of problems with Heritage's report, saying the methodology it uses to count costs is flawed.

Most importantly, they don't view immigration reform through the "makers versus takers" lens. They believe reform that legalizes the undocumented could provide significant economic benefits, which they say Heritage has downplayed or outright ignored.

Haley Barbour, the former GOP governor of Mississippi, put it more bluntly.

"It's a political document," he told reporters on a conference call. "It's not serious analysis."

For example, pro-immigration reform conservatives say the Heritage study does not adequately account for the rising wage levels for immigrants who, say, go from being a dishwasher to owning a restaurant as a result of gaining legal status. They also say that education levels, especially among younger undocumented immigrants, could rise dramatically if they adjust their status.

Higher education levels could lead to higher paying jobs. And higher-paying jobs mean less welfare benefits and more tax dollars for the federal government. One study published by the libertarian Cato Institute last year said that, taking those factors into account, comprehensive immigration reform could add $1.5 trillion to the U.S. gross domestic product over ten years.

Even though the report has serious issues, the political consequences of this "cost" debate could help determine whether the bill advances with bipartisan support, or dies on the vine because of a Republican revolt.

When the last immigration reform bill was released in 2007, the Heritage Foundation published a similar study that claimed it would cost taxpayers $2.6 trillion. That helped fuel conservative outrage against the proposal and ultimately it failed in Congress.

Cognizant of that, pro-immigration reform conservatives have mobilized to counter Heritage's study. Grover Norquist's Americans for Tax Reform (ATR) circulated a "pre-buttal" memo to top Republican aides on Capitol Hill last month that criticized the work of Robert Rector and the Heritage Foundation.

"The Heritage Foundation is a treasured ally in the conservative movement and a pillar of the conservative policy community," Joshua Culling, ATR's top immigration policy aide, said in a statement on Monday. "However, this study is every bit as flawed as its 2007 iteration."

"This study is designed to try to scare conservative Republicans into thinking the cost is going to be so gigantic [they can't support it]," Barbour said. "The opponents of immigration reform think ... they can try to redo the same tactics and strategies [they used six years ago]."

All of this might seem like inside baseball. But it's esoteric debates like this one that could eventually determine the fate of some 11 million people.